The basic objective of this project is to analyze the capacities of primitive mammalian hematopoietic progenitor cells to undergo self-renewal and differentiation in relation to their position within the cell cycle. By employing cell separation techniques, both with centrifugation and at unit gravity, repidly growing murine bone marrow will be sedimented to obtain fractions enriched in pluripotent and unipotont cells (those responding to the hormone erythropoietin). Proliferating stem cells in each cell-cycle phase, including Go cells will thus be obtained and their capacity to undergo self-renewal tested by double in vivo spleen colony transplant experiments. The capacity of spleen colonies (CFU) for differentiation will be tested by the in vitro CFC-agar assay method, as well as the in vitro assay for cells committed to erythropoiesis (i.e., the plasmaclot method for CFU-E). Both self-renewal and differentiation capacities will be assessed in relation to the position of CFU within the cell cycle. Unipotent cells (CFU-E) will likewise be synchronized by sedimentation and the capacity of these cells to undergo erythroid differentiation assessed as a function of their cell cycle. These experiments are expected to provide insight into the relationship of progenitor cells to one another and into the nature of capacity of progenitor cells for differentiation and self-renewal in relation to the cell cycle.